1. Field of the Invention
The invention relates to the separation and recovery of ethylene and hydrogen from hydrocarbon feed mixtures. More particularly, it relates to the avoiding of acetylene solidification during said separation and recovery operations.
2. Background of the Invention
The chemical processing industry has long recognized the significant problems that are created as a result of acetylene solidification in hydrocarbon separation systems, e.g., in an ethylene plant. Not only does such solidification create plugging problems in cold process piping and equipment, but the instability of solid acetylene creates a very hazardous and explosive environment within the system. Extreme care must be exercised, therefore, to avoid any potential for acetylene solidification in such hydrocarbon separation systems.
In many cases, ethylene plants and other hydrocarbon separation systems are designed to remove acetylene at the warm end of said systems. Typical operations for this purpose employ either catalytic hydrogenation of the acetylene content of hydrocarbon feed mixtures over a platinum or palladium catalyst, or a physical or chemical absorption of said acetylene, employing a variety of suitable solvents, e.g., methanol, acetone or dimethalformamide (DMF). Additional information concerning ethylene plants utilizing either of these acetylene removal techniques is available in numerous prior art references, such as the following U.S. Pat. Nos.: 2,818,920--Cobb; 2,915,881--Irvine; 3,095,293--Kuerston; 3,187,064--Wang et al and 4,167,402--Davis. In other cases, cracked gases may be produced under furnace cracking conditions such that the quantity of acetylene formed is low enough to permit its safe handling without the necessity for employing such processing techniques in the warm end of hydrocarbon separation systems. Depending upon the particular feedstock employed and the operating conditions prevailing in the cracking furnace, however, significant quantities of acetylene may nevertheless be present in the hydrocarbon gas stream delivered to the cold end of the hydrocarbon separation system even in such cases intended to minimize the acetylene content of said stream. In such instances, the processing of the hydrocarbon gas stream at the cold end of the separation system must be appropriately regulated to avoid the solidification of the acetylene content of said stream.
Recognizing the seriousness of the potential for acetylene solidification in the cold end of hydrocarbon separation systems, the art has proposed several alternative approaches for solving this problem. In one such approach, the ethylene concentration of the feed gas is monitored and regulated by the injection of pure ethylene therein, as needed, to insure that sufficient ethylene is present in any of the subsequently condensed liquid phases to serve as a solvent for any acetylene that might otherwise solidify during the processing of the hydrocarbon feed gas.
The problem of acetylene solidification was also recognized in the Danneil et al patent, U.S. Pat. No. 3,607,963 specifically with regard to a process designed to recover acetylene from a gas obtained from the cracking of petroleum or petroleum fractions by a flame burning beneath the surface thereof. In this process, the cracked gas is cooled by countercurrent heat exchange with warming streams recovered from the cracked gas. The gas stream is first cooled to a temperature above the solidification temperature of acetylene when in admixture with other condensible components of the cracked gas. This cooling results in the condensation of a liquid fraction that contains a large portion of the ethylene content of the cracked gas. The condensed liquid fraction is then separated from the uncondensed gas as a product stream. The total uncondensed gas stream is then freed from acetylene therein by washing such gas with a liquid consisting of ethane, ethylene or a mixture thereof. The gas mixture, essentially free of acetylene, is then further cooled to condense substantially all of the remaining fraction of ethylene in the thus-treated cracked gas feed. The condensed liquid is then removed as a product, which can be further treated to recover the ethylene content thereof. The uncondensed gas fraction is work expanded to low pressure to develop process refrigeration. By this process of Danneil et al, the potential for acetylene solidification during the work expansion step is said to be essentially eliminated.
The problem of acetylene solidification can also be avoided in the operation of many ethylene fractionation systems at high superatmospheric pressures, i.e., above about 370 psia (25 atmospheres). By operating at such high pressures, acetylene solidification can generally be avoided in the ethylene recovery section of the cold end of a hydrocarbon separation system. Thus, enough acetylene is generally removed with the condensed hydrocarbon liquid fractions at such high pressures, so that very little acetylene is left in the uncondensed gas fraction that is subsequently work expanded to low pressures to develop refrigeration for the process. Under such circumstances, the acetylene wash system disclosed in the Danneil et al patent is not generally needed to prepare the cracked gas for ethylene recovery in the cold end of the hydrocarbon separation system, unless the process is operated at relatively low superatmospheric pressures.
Such use of high superatmospheric pressures for ethylene recovery does not, however, remove acetylene solidification as a problem in hydrocarbon separation systems. To the contrary, the hydrogen recovery section of the cold end of a hydrocarbon recovery system can not be conveniently operated under appropriate conditions so as to avoid acetylene solidification although, as indicated above, it is possible to operate the ethylene recovery section so as to avoid such undesired acetylene solidification. In order to successfully produce a high purity hydrogen stream, essentially all of the less volatile components of the cooled cracked gas must be removed from the hydrogen component. This requires the use of extremely low temperatures, generally below about -150.degree. C. (123.degree. K.). Such low temperatures are provided by reboiling the separated liquid fraction at a low pressure, generally below about 60 psia (4 atmospheres), oftentimes with the admixture of a small amount of product hydrogen to further reduce the reboil temperature. Unless the amount of acetylene present in the separated liquid fraction is quite low, the acetylene will likely solidify when this liquid is subsequently throttled to the low pressure. Because of the low pressure and low temperatures involved in the hydrogen recovery section of the system, the threshold acetylene concentration that can be tolerated in the separated liquid is very low.
The separation and recovery of ethylene and hydrogen from hydrocarbon feed mixtures can thus be seen as an operation concerning which improvement would be desirable in the art. While the use of certain processing conditions as taught in the art may serve to reduce the problem of acetylene solidification, it will be seen from the above that the presence of acetylene in the various streams processed in the cold end of the separation system presents a genuine potential for serious processing difficulties in commercial practice. The alternative approaches of acetylene control or removal as discussed above, i.e., the injection of pure ethylene into the monitored feed gas to act as a solvent for the acetylene content of the feed gas or the Danneil et al process for washing the uncondensed gas to remove said acetylene therefrom, provide possible solutions that are not entirely satisfactory in practical commercial operations. The capital and operating costs associated with such approaches represent additional expenses solely for acetylene treatment, thus tending to reduce the practical feasibility of the overall separation process. The necessity for treating the hydrocarbon feed stream or the overall uncondensed gas fraction because of the potential for acetylene solidification problems, which differ in different sections of the cold end of the hydrocarbon recovery system, constitutes one aspect of the disadvantages associated with such prior art solutions to the problem of acetylene solidification in a hydrocarbon recovery system.
It is an object of the invention, therefore, to provide an improved process for avoiding the solidification of acetylene in hydrocarbon separation systems for ethylene and hydrogen separation and recovery.
It is another object of the invention to provide a process for ethylene and hydrogen recovery from hydrocarbon feed mixtures in which the amount of gas treated to avoid acetylene solidification is minimized.
It is a further object of the invention to provide a process for eliminating the potential for acetylene solidification in ethylene recovery plants, while minimizing the capital and operating costs associated with such acetylene treatment operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.